1. Field of the Invention
The present invention relates to a front scattering film used for reflection type liquid crystal display units. In particular, the present invention relates to a front scattering film of improved optical characteristics, which is used for liquid crystal display units using a mirror reflection board as a reflective layer.
2. Related Art
Reflection type liquid crystal display units suffer from drawbacks such as lower contrast and darker display than conventional transmission type liquid crystal display units utilizing back light. However, they also have advantageous features such as thin shape, lightweight, and low electric power consumption, since they do not require aback light source in their structure. These characteristics constitute the major reason for the use of reflection type liquid crystal display units as display devices of, in particular, portable information terminals such as notebook computers, and they have attracted attention for their high utility.
The drawbacks of the reflection type liquid crystal display units, such as low contrast and dark display, mainly originate from low light utilization efficiency, and several means have been proposed to solve this problem.
Among those, the reflection type liquid crystal display unit disclosed in Japanese Patent Unexamined Publication (KOKAI) No. 8-201802/1996 has a structure comprising a mirror reflection electrode, liquid crystal layer, transparent electrodes, color filter, and transparent substrate stacked in this order, on which a birefringence film, polarizer and front scattering board are further stacked in this order, and it is a useful reflection type liquid crystal display device because it exhibits high contrast and does not suffer from problems of viewing angle dependency, double images (real image and shadow thereof) and the like.
Further, Ishinabe et al. (Shingaku Giho, 02, 125-130 (1996)) reported about some parameters using Mie""s scattering theory and Hartel""s theory, which parameters may be guideposts for designing a light scattering film (front scattering board) comprising spherical microparticles dispersed in a transparent medium and being suitable for a reflection type liquid crystal display unit called the R-OCB type.
However, the parameters reported in the aforementioned article do not necessarily enable design of a front scattering film having sufficient display characteristics such as performance of exhibiting paper-like whiteness in white state or blacker in black state, high contrast of image and no blurring.
Moreover, proper production control is difficult only with these several parameters. Therefore, even if a front scattering film showing paper-like white in white state, blacker in black state, high contrast or image and no blurring could be obtained, it would be extremely difficult to confirm whether it can actually exhibit sufficient visibility.
The object of the present invention is to provide a front scattering film that can exhibit excellent visibility including paper-like whiteness in white state, more blackness in black state, high contrast of image display, no blurring and the like, and realize extremely easy production control thereof.
As a result of the present inventors"" studies for achieving the aforementioned object, it was found that a front scattering film having the characteristics mentioned below can exhibit an excellent visibility, including paper-like whiteness in white state, more blackness in black state, high contrast of image display, no blurring and the like.
Specifically, a front scattering film according to the present invention has a light scattering layer consisting of transparent polymer binder containing spherical microparticles, wherein the spherical microparticles have a mean particle diameter of 1.0 xcexcm to 10.0 xcexcm, and a refractive index (n) relative to the refractive index of the transparent polymer binder satisfying a condition of 0.91 less than n 1.09, and the front scattering film has a haze (cloudiness value) of 30.0% or more and a distinctness of image of 60.0% or more.
In another preferred embodiment of the aforementioned front scattering film of the present invention, at least one surface of the light scattering layer has a transparent resin film.
In a further preferred embodiment of the aforementioned front scattering film of the present invention, at least one surface of the film has adhesiveness.
In a still further preferred embodiment of the aforementioned front scattering film of the present invention, the transparent polymer binder comprises an adhesive.
The term xe2x80x9chazexe2x80x9d used with respect to the present invention means a haze value determined in accordance with JIS-K7105, which can be calculated according to the equation:
H=Td/Ttxc3x97100 [H; haze, Td: diffused light transmittance, and Tt: total light transmittance].
The term xe2x80x9cdistinctness of imagexe2x80x9d used with respect to the present invention means a value of distinctness of image determined in accordance with JIS-K7105, and it can be obtained by measuring the maximum wave height [M] and the minimum wave height [m] at an optical comb of 0.125 mm using the transmission method, and calculating according to the following equation:
Distinctness of image [C(0,125)]={Mxe2x88x92m}/{M+m}xc3x97100 (%).
The value of distinctness of image used for the present invention is an average of values measured along the longitudinal direction and the transverse direction for each sample.
According to the present invention, there is provided a front scattering film that can exhibit excellent visibility including paper-like whiteness in white state and more blackness in black state, high contrast of image, no blurring and the like.
According to the present invention, there is also provided a front scattering film that realizes extremely easy production control thereof.